The Hunter River within in the subregion is regulated by the Glenbawn and Glennies Creek dams. While the AWRA-R modelling of the regulated river includes some aspects of river regulation, it was never intended to be a river operations model, and does not include the more comprehensive set of rules for representing river regulation in the Hunter River that is part of NSW Department of Primary Industries Water Hunter IQQM. The Hunter AWRA-R model uses a simplified approach to represent dam releases based on current levels of demand, ensures that minimum environmental water requirements are met, and includes some rules for mine water discharges under the Hunter River Salinity Trading Scheme (see companion product 2.6.1 (Zhang et al., 2018) and companion product 2.1-2.2 (Herron et al., 2018a) for the Hunter subregion for details of the AWRA-R implementation). Thus the results summarised below do not assume any changes in dam operations in response to the modelled changes.

At the 95th percentile, 408 km of the stream network in the surface water zone of potential hydrological change is modelled to experience increases of at least 3 days in the number of low-flow days per year (as per definition in Table 6). A further 801 km of non-modelled streams could experience similar increases in low-flow days due to additional coal resource development as they flow through catchments disturbed by open-cut mining or are in the groundwater zone of potential hydrological change (Table 11).

Where changes have been quantified, about 22 km of streams in the Central Hunter, Lower Hunter and the Macquarie-Tuggerah reporting areas are very likely (5th percentile) to experience increases of at least 3 days per year. They include parts of the Wyong River, Saddlers Creek, Loders Creek, Dry Creek, Swamp Creek and three unnamed creeks draining the Mount Pleasant, Mount Thorley–Warkworth and Liddell additional coal resource developments. There is at least a 5% chance that increases in low-flow days in these six streams will exceed 200 days per year due to additional coal resource development.

In the ‘potential hydrological change’ reaches where streamflow changes due to the additional coal resource developments have not been quantified, an indication of the potential increases in low-flow days can be inferred from nearby model nodes and stream reaches where potential increases have been quantified, having regard to stream order and proximity to additional coal resource developments.

Increases of more than 80 days per year are very likely in the small unnamed creek which drains the northern side of the Mount Pleasant development; and more than 20 days per year in Loders Creek, which drains the Mount Thorley–Warkworth and Bulga developments, and Dry Creek which drains the southern side of the Bengalla development. As identified in companion product 2.6.1 for the Hunter subregion (Zhang et al., 2018), these potentially large changes reflect the fact that the mine footprints in these catchments are a large proportion of the total contributing area.

The potentially large changes in some of the small tributary streams of the Hunter River are relatively localised. The much larger Hunter River is not particularly sensitive to the modelled changes in inflows from these tributaries because of its much greater volume of flow and because its flow can be augmented with releases from storage. On the Hunter River, downstream of its junction with Saltwater Creek to just upstream of Singleton, there is a 5% chance of increases of more than 80 days per year in low-flow days. These changes likely reflect the more extreme groundwaterdrawdown results and associated reductions in baseflow, rather than sensitivity of the river to changes in tributary inflows.

In the Wyong River, results of the modelling indicate a risk of potentially significant impacts from the proposed Wallarah 2 and Mandalong Southern Extension developments. Using the full set of regional parameters, changes in low-flow days of more than 200 days per year are possible (5% chance), which is outside the range of previously experienced low-flow days due to interannual variability. Jilliby Jilliby Creek, which flows through the Wallarah 2 mine lease area and into the Wyong River, and Dora Creek, which drains from the Mandalong Southern Extension, were not represented in the surface water modelling, but could experience similar changes in flow regime to those modelled for Wyong River due to the potentially large drawdowns predicted in the groundwater modelling (Section 3.3.2)

The Wyong River results require further investigation for a number of reasons:

Unlike the other streams identified from the regional-scale assessment as ‘at risk’ of potentially large increases in the number of low-flow days per year, the Wyong River is a perennial river, draining a comparatively larger area, and part of the water supply to the Wyong-Gosford area.

Unlike the other ‘at risk’ streams, which are located close to baseline developments, it is essentially a ‘greenfield’ area, as hydrological changes from the baseline development at Mandalong in the adjoining Dora Creek catchment are unlikely to have had a significant effect on flows in the Wyong River and its tributary Jilliby Jilliby Creek (see upper panel in Figure 14).

Unlike the other ‘at risk’ streams, which have been assessed as being in poor condition and low recovery potential, the Wyong River has been assessed as being in moderate condition with rapid recovery potential (NSW Office of Water, Dataset 10).

The Wallarah 2 development, which is proposed in this catchment, has been controversial due to concerns about potentially adverse impacts on town water supply and ecologically important vegetation communities and habitat.

Given the potentially greater level of risk in the Wyong River catchment, local hydrogeological information was obtained from the Wallarah 2 Environmental Impact Statement (Mackie Environmental Research, 2013) to constrain the groundwater modelling results to the sub-set of simulations (from the full set run as part of the regional-scale assessment) with parameter values consistent with that information (see Section 2.6.2.8 of companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b)). The effect on drawdown predictions of using local information, which indicated that median hydraulic conductivities in this area are about two orders of magnitude lower than the median based on the regional dataset, is shown in Figure 47 of companion product 2.6.2 for the Hunter subregion (Herron et al., 2018b). The potentially affected area based on a 0.2 m drawdown threshold is smaller across all percentiles (Table 10), and the hydraulic gradient within this area is less steep.

The implications for baseflow to streams and the number of low-flow days per years are dramatic. At the 5th and 50th percentiles, results suggest that the additional coal resource development (predominantly from Wallarah 2, but potentially also affected by the Mandalong Southern Extension) will not have any effect on the number of low-flow days, and that it is very unlikely that the increase in the average number of low-flow days per year would exceed 7 days.

To understand the significance of the modelled increases in low-flow days, it is useful to look at them in the context of the interannual variability in low-flow days due to climate. In other words, are the modelled increases due to additional coal resource development within the natural range of variability of the longer-term flow regime, or are they potentially moving the system outside the range of hydrological variability it experiences under the current climate? The maximum increase in the number of low-flow days due to additional coal resource development relative to the interannual variability in low-flow days under the baseline has been adopted to put some context around the modelled changes. This ratio is shown qualitatively for each surface water model node in Figure 23. Table 12 provides the ratio ranges for LFD, FD and AF adopted for each qualitative ratio class shown in Figure 23. It is important to be aware that the changes shown in Figure 23 represent the maximum change due to additional coal resource development in a single year relative to the interannual variability across 90 years under the baseline. Thus it is not a comparison of distributions, but an assessment of whether the change due to additional coal resource development, in the year of maximum difference between the CRDP and the baseline, is within the range of natural variability. If the maximum change is small relative to the interannual variability due to climate (e.g. an increase of 3 days relative to a baseline range of 20 to 50 days), then the risk of impacts from the changes in low-flow days is likely to be low. If the maximum change is comparable to or greater than the interannual variability due to climate (e.g. an increase of 200 days relative to a baseline range of 20 to 50 days), then there is a greater risk of impact on the landscape classes and assets that rely on this water source. Here changes comparable to or greater than interannual variability are interpreted as presenting a risk. However, the change due to the additional coal resource development is additive, so even a ‘less than interannual variability’ change is not free from risk. Results of the interannual variability comparison should be viewed as indicators of risk.

Table 12 Ratio of increase in the number of low-flow days (LFD), high-flow days (FD) and annual flow volume (AF) due to additional coal resource development to the interannual variability in low-flow days under the baseline

Qualitative ratio class

Ratio range

No significant change

LFD <3 days

FD ≥3 days

AF ≥1%

Less than interannual variability

<0.5

Comparable to interannual variability

0.5–1.5

Greater than interannual variability

>1.5

FD = high-flow days – in previous products, this is referred to as ‘flood days’

At the 5th percentile (Figure 23, top left), the modelled changes in low-flow days represent no significant change or are less than the interannual variability at all model nodes, except one. The change in low-flow days on the unnamed stream near the Mount Pleasant development is very likely to experience a change that is comparable to the interannual variability under the baseline, which indicates a potential risk to water-dependent landscape classes and assets in this vicinity. At the 50th percentile (Figure 23, main panel), the changes at five model nodes – in the Central Hunter, Lower Hunter and Macquarie-Tuggerah reporting areas – are comparable to or exceed the baseline interannual variability, suggesting major changes in flow regime driven by reduced runoff and weaker connections to regional groundwater. At the 95th percentile (Figure 23, top right), the increases in low-flow days at 17 locations across the assessment extent suggest the possibility of widespread flow regime changes, particularly in unregulated streams where river flows cannot be topped up through releases of dam water. As discussed above for the Wallarah 2 mining area, local-scale information is needed to refine the regional-scale estimates in areas identified as at risk. The modelled changes in low-flow days in the Wyong River based on the constrained set of model simulations suggest that any changes in the number of low-flow days due to additional coal resource development are likely to be well within the interannual range due to climate in this area.

As stated in Table 6, a high-flow day is defined as one in which the streamflow exceeds the 90th percentile flow from the simulated 90-year period (2013 to 2102) for that stream. Reduction in the number of high-flow days due to additional coal resource development in the Hunter subregion is shown in Figure 24, based on results from the regional parameter set. Reductions in high-flow days of at least 3 days per year are very likely along lower Wollar Creek, which drains the Moolarben and Wilpinjong mine developments, and in four of the five streams identified as very likely to experience above-threshold increases in low-flow days (Figure 22). There is at least a 50% chance that the Wyong River will experience reductions in high-flow days of at least 3 days per year, but is very unlikely to experience reductions greater than 20 days per year. However, when the result set is constrained to those simulations with parameter values that are consistent with local hydrogeological information, it appears more likely that the Wallarah 2 development will have a negligible effect on high flows in the Wyong River. It is very unlikely that the Hunter Regulated River and most of the Goulburn River will experience reductions in high-flow days of more than 10 days per year.

The total length of stream potentially impacted by reductions in high-flow days is 1116 km, of which the magnitude of change is quantified for 470 km at the flow class level and not quantified for 646 km (Table 13).

The comparison of maximum change in high-flow days due to the additional coal resource development and interannual variability in high-flow days under the baseline (Figure 25) shows that at most nodes, the maximum change is relatively small compared to interannual variability and that the modelled changes are unlikely to increase the stress on these streams. Two streams near Mount Pleasant and Bengalla mines (near Muswellbrook) and in the vicinity of the Mount Thorley–Warkworth and Bulga mines (near Singleton) could potentially experience reductions in high-flow days outside the interannual variability under the baseline. Generally, the impact of additional coal resource development on high-flow days is not as great as it is on low-flow days. In particular, the decrease in number of high-flow days in Saddlers Creek due to the Mount Arthur and Drayton South developments and in the Wyong River due to the Wallarah 2 and Mandalong Southern Extension developments are noticeably less than the increase in number of low-flow days (see Figure 22 and Figure 23).

Decreases in mean annual flow of at least 5% are very likely in lower Wollar Creek, Saddlers Creek, Loders Creek, Dry Creek, Swamp Creek and the three unnamed creeks draining Mount Pleasant, Mount Thorley–Warkworth and Liddell additional coal resource developments, corresponding to 271 km of stream length where results from model nodes can be interpolated to flow classes. Changes in mean annual flow of at least 5% potentially occur more extensively as some of the non‑modelled ‘potential hydrological change’ stream reaches near mining operations are likely to be impacted to a similar degree.

Decreases in mean annual flow of at least 50% are very likely in Loders Creek, Dry Creek, Swamp Creek and the unnamed creeks draining Mount Pleasant consistent with the potentially large reductions in high-flow days and shift to greater frequency of low-flow days. These changes are localised as these relatively minor streams feed into the much larger Hunter River, which is largely insensitive to these changes in inflows.

Decreases in mean annual flow of at least 1% are very likely along part of the Goulburn River and the Hunter Regulated River, downstream of Saltwater Creek, but decreases of more than 5% are very unlikely.

Using the regional parameter set, the effect of additional coal resource development on mean annual flow in the Wyong River is predicted to not be significant (<1% reduction) at the 50th percentile, but there is at least a 5% chance of reductions between 1% and 5% of the baseline mean annual flow. The potentially small effect on mean annual flow relative to the potentially large effect on low-flow days reflects the fact that mean annual flow is strongly influenced by high flows in the river, and that while a small reduction in baseflow to a stream can have a big effect on the number of low-flow days, this does not necessarily result in a big change in annual streamflow volumes. As discussed previously, when the constrained set of simulations is used to assess risk of potentially adverse impacts in the Wyong River, the potential reductions in mean annual flow range from <0.2 GL/year (5th percentile, 2013 to 2042) to about 1.25 GL/year (95th percentile, 2043 to 2072), well below the range predicted using the regional parameter set (0.2 to 5.7 GL/year).

The maximum change in annual flow due to additional coal resource development relative to the interannual variability of annual flow under the baseline is shown for each surface water model node in Figure 27. In no case is the maximum change in annual flow due to additional coal resource development greater than the interannual variability under the baseline. There is at least a 50% chance at four locations and at least a 5% chance at another two locations that the changes are comparable to the interannual variability under the baseline. These occur in the Central Hunter and Lower Hunter reporting areas only.